OLED (Organic Light-Emitting Diode) as a current type light emitting device has been more and more applied to high performance display. Conventional passive matrix OLEDs need shorter driving time for a single pixel with the increase of display size, and thus require increased instantaneous current; therefore power consumption is increased. At the same time, the application of large current causes too large voltage drop in the ITO (Indium Tin Oxide) line, and causes too large operating voltage of the OLED, reducing its efficiency. On the contrary, the AMOLED (Active Matrix OLED) can solve those problems well by scanning current inputted into OLED line by line via switch transistors.
In backboard design of AMOLED, the main problem needing to be solved is the brightness nonuniformity between pixels.
First, AMOLED uses TFTs (Thin Film Transistors) to construct a pixel circuit to provide corresponding current for the OLED, wherein low temperature polycrystal silicon TFTs (LTPS TFTs) or oxide TFTs are mostly used. Compared with a general amorphous silicon TFT (amorphous-Si TFT), the LTPS TFT and the oxide TFT have larger mobility and more stable characteristics, and are more suitable for be applied to AMOLED display. However, due to the limitation of crystallization process, the LTPS TFTs fabricated on a large glass substrate usually have nonuniformity on electrical parameters such as threshold voltage, mobility and so on. Such nonuniformity will be converted to current difference and brightness difference of the OLED display devices which can be perceived by human eyes, i.e., a mura phenomenon. Although the Oxide TFT has good uniformity in terms of process, similar to the a-Si TFT, its threshold voltage will drift when voltage and high temperature are applied for a long time. TFTs in different parts of the panel have different drift amount due to different display pictures, which would cause display brightness difference. Because this difference is related to the picture displayed previously, it is usually presented as an afterimage phenomenon.
Second, in large size display applications, since the power supply lines of the backboard have resistance and the driving currents of all pixels are provided by a first reference voltage source VDD, the power source voltage at the area near the power supply position of the first reference voltage source VDD in the backboard is higher than the power source voltage at the area far away from the power supply position. This phenomenon is called IR Drop. Since the voltage of the first reference voltage source VDD is related to the current, the IR Drop would cause different currents at different areas, then creating mura during display. The LTPS process using P type TFTs to construct pixel units is particularly sensitive to this issue, because its storage capacitor is connected between the first reference voltage source VDD and the gate of the driving transistor DTFT, and thus the voltage change of the first reference voltage source VDD would directly influence the gate-source voltage Vgs of the driving transistor DTFT.
Third, the OLED device would have nonuniformity of electrical performance due to nonuniform film thickness during evaporation. For the a-Si or Oxide TFT process using N type TFTs to construct pixel units, its storage capacitor is connected between the gate of the driving TFT and the anode of the OLED. When the data voltage is transferred to the gate, if the anode voltages of the OLEDs of respective pixels are different, the gate-source voltages Vgs actually applied on the driving transistor DTFTs are different, such that the difference of driving currents causes display brightness difference.
Normally, an AMOLED voltage type pixel unit driving circuit is provided. Such a voltage type driving method is similar to the conventional AMLCD driving method, in which a driving unit provides a voltage signal representing a gray scale, and the voltage signal is converted into a current signal for a driving transistor inside the pixel circuit, so as to driving the OLED to realize the gray scale of brightness. Such a method has advantages of fast driving speed and easy realization, and is suitable for driving a panel with large size and is widely used in the industry. However, it is needed to design additional switch transistors and capacitor devices to compensate the nonuniformity of driving transistors DTFTs, the IR Drop and the nonuniformity of the OLEDs.